The use of a metallized plastic strip embedded within currency paper as a security thread for counterfeit deterrence is described within U.S. Pat. No. 4,652,015 and 4,761,205, which Patents are incorporated herein for purposes of reference. The security thread is virtually undetected under reflected light and legible under transmitted light to verify its presence.
In commercial situations where verification of currency bills is required, the receiver of the currency bill should subject the currency to a relatively intense light source to read the security thread under transmitted light. With large queues of customers at a bank or supermarket, as well as in places of low level illumination such as bars and restaurants it is difficult to visually inspect the corresponding large number of currency bills. It would be advantageous therefore to have some means of automatically determining the presence of the requisite security thread and confirming authenticity to the teller or cashier.
U.S. Pat. 3,980,990 entitled "Ferromagnetic Currency Validator" describes a magnetic detection circuit which first submits a proffered currency paper to a magnetic source to magnetize the ferromagnetic ink used with the signature on the portrait surface of the bill.
U.S. Pat. No. 4,524,276 entitled "Apparatus for Detecting a Security Thread Embedded in a Paper-Like Material" describes an infrared radiation source and two infrared radiation detectors used to determine whether or not a security thread is embedded in the paper-like material and also what the detected security material is made of.
Countries outside of the United States that employ plastic or metal security threads embedded in their paper currency, require that the presence of such security threads be ascertained under transmitted light such as described in the aforementioned U.S. Pat. No. 4,524,276. In accordance with the United States requirement that the currency security thread be detected under transmitted light and not seen under reflected light, both reflective and transmissive determinations must be made for complete verification of the currency.
U.S. Pat. No. 4,980,569 describes a security paper verification device wherein optical means are arranged on opposing surfaces of the currency to determine the absence of any device on the surface of the currency while detecting the presence of the device within the currency. This is to prevent attaching counterfeit security threads to the outside surface of the currency while ascertaining that the genuine security thread is embedded within the currency.
U.S. patent application Ser. No. 694,912 filed May 2, 1991 entitled "Currency Verification Device" describes combination of optical means with inductive or capacitive sensors for verifying the presence of the security thread in currency paper.
In some applications, a simple infrared transmissive photodiode and photodetector could be used to indicate the presence of the security thread within currency after a visual inspection of the currency surface to insure that the currency appears genuine.
In other applications, the magnetic sensor could provide indication of the currency denomination by quantitizing the amounts and locations of the magnetic oxides used with the different denominations and using such denomination information for automated currency sorting.
Accordingly, one purpose of the invention is to provide a simple security device that will optically detect a genuine security thread within the currency paper after optical inspection of the outer surface of the currency paper. An additional purpose of the invention is to describe customized magnetic and capacitive sensors that respond to both the metal materials used to form the security thread within the currency as well as the magnetic oxides used with the currency signatures to provide verification of the presence within the currency after such visual inspection of the currency surface. Additionally, the invention proposes to determine the currency denomination and to provide such denomination information to a currency receiver and/or to currency sorting equipment.